The invention relates to a device for controlling the operation of a vehicle battery.
Although the battery in a motor vehicle represents a known weak point from the viewpoint of reliability, at present there is still no dependable and inexpensive device available for monitoring the state of the battery. This is due above all to the fact that the battery state that governs the starting ability of the vehicle is dependent on a large number of factors such as, for example, temperature, voltage, current load when charging and discharging, and the age of the battery. As a result, efforts were made to take account of these factors in very complex and involved calculations. Therefore, it was proposed among other things that the variation in the voltage during starting be monitored and different empirical data be incorporated in the evaluation.
German Patent No. DE 35 20 985 C2 discloses a device for monitoring the state of charge of the starter battery of a motor vehicle which continuously measures the battery current and the battery voltage and integrates the measured current to determine the lost charge. In the currentless state of the battery, the device detects the settling equilibrium voltage and calculates a hypothetical battery capacity from this equilibrium voltage and the charge previously extracted while in operation. A measurement for the instantaneous state of charge of the battery is calculated from the charge extracted when the battery current is flowing again and the calculated hypothetical battery capacity according to a predetermined formula.
German Patent Application No. DE 43 41 826 A1 teaches a method for determining the state of charge of an electrical energy store from which a starter motor of an internal combustion engine influenced by a control device and/or electrical consumers are supplied with electrical energy. In order to be able to describe the state of charge of the electrical energy store as accurately as possible, it is proposed in a first variant of the method that the voltage present at the energy store and the current flowing be detected during the starting operation. In the process, the current in the control device is differentiated according to the time. When relative maxima occur, the appropriate voltage value is detected and a magnitude characterising the state of charge of the energy store is derived from this. According to a second variant of the method, the variation in time of the internal resistance of the energy store is calculated through a series of successive synchronised current and voltage measurements and this is used to determine the instantaneous state of charge of the energy store on the basis of a reference characteristic stored in the control device.
All the methods of this kind require very accurate detection of the measurement data and complex data processing due to the problem of the accumulation of measuring errors over lengthy periods of time.
Therefore, an underlying object of the invention is to create a device for controlling the operation of a vehicle battery which, without major intervention in the vehicle electrical system, allows reliable and accurate monitoring of the state of the battery and thus an indication of the starting ability of the battery for the vehicle user.
The present invention relates to a device for controlling the operation of a vehicle battery and includes two current paths for connecting the battery to the vehicle electrical system. The first current path allows current to flow in both directions and the second current path only allows a charging current to flow from the vehicle electrical system to the battery. In addition, the first current path contains a switch which is only closed when the voltage at the generator connection is less than a predetermined value.
In other embodiments, a measuring device which measures a change in the state of charge of the battery, a storage device which stores the instantaneous state of charge of the battery, and a checking device which checks a predetermined electrical criterion for the maximum attainable state of charge of the battery, are provided. The instantaneous state of charge and the attainment of the maximum attainable state of charge are indicated to the user of the vehicle by an indicating device which also provides information about the minimum possible starting temperature in the instantaneous state of the battery.
The invention ensures that the vehicle battery reaches the state of maximum charge, referred to hereinafter as the fully charged state, as rapidly as possible in operation. This greatly simplifies the object of monitoring the state in that in said fully charged state the starting ability is now dependent solely on parameters which are simple to detect such as the ambient temperature and the service age of the battery. As long as the fully charged state does not apply, which is the case in particular shortly after a starting operation or after use of energy-consuming devices while the engine is switched off, the charge balance resulting from consumption and charging is included in the state of the battery and must be taken into account when determining the starting ability.